As I mentioned before, the 1.5" sch 10 runners are the exact dimensions of the Steedspeed manifold internally(they are actually ~1.6" inside diameter ports). I do believe you will make more power with this full tubular manifold. Fitting a 6-1 tubular manifold with the 1.5" schedule 10 pipe is a pain due to the bigger bend radius. We use 1.25" sch 10 pipe on most of the e36 manifolds. My car does have 1.5" sch 10 primaries now.